1. Field of the Invention
The present invention relates generally to electrical power conversion, and more particularly, to switching-mode power converters having complementary synchronous rectification.
2. Description of the Related Art
DC (Direct Current)-to-DC power converters are commonly used to convert power from one DC level to another. FIG. 1 shows a simplified single-ended converter signified by the reference numeral 2. The converter 2 includes a transformer 4 having a core 5. There are primary and secondary windings 6 and 8 wound around the core 5. The primary winding 6 is connected to a DC power source 3 through a switch 7. Attached across the secondary winding 8 is an inductor 10 connected in series with a capacitor 12 via a rectifier 9. In this case, the rectifier 9 is a diode 14. There is also a free-wheel diode 15 connected across the secondary winding 8 as shown in FIG. 1.
The operation of the converter 2, in conjunction with some basic terms relating to rectification are herein described. Reference is now directed to FIGS. 1 and 2. Suppose the DC power source 3 supplies a DC voltage VIN. The switch 7 is turned on and off periodically. As a consequence, a periodic step-pulse voltage vP is generated and is applied across the primary winding 6. In this specification, lower case alphabets are used to denote parameters that vary with time. For example, vP designates a time-varying voltage signal. During the first half-cycle t1 (FIG. 2), the primary voltage vP is positive, from basic electromagnetic theory, a positive secondary voltage vS is induced in the secondary winding 8. The positive-going secondary voltage vS forward biases the rectifying diode 14. Consequently, the capacitor 12 is charged via the forward-biased diode 14 through the inductor 10. The resultant current path is denoted by the reference numeral 11, as shown in FIG. 1. The converter 2 is said to be in a forward rectification mode.
At the end of the half-cycle t1, the supply voltage vP begins to switch polarities and approaches the zero potential. However, at this juncture, the stored energy in the transformer, such as in the windings 6 and 8, releases and sends spurred signals of opposite polarity to original voltage vs. Take the secondary winding 8 as an example. The spurred signal is in the form of a spike 16 as shown in FIG. 2. Phrased differently, in accordance with Lenz's law, the sudden cessation of current supply is in the secondary winding 8 provokes the winding 8 to generate a voltage spike 16 of opposite polarity to that of the secondary voltage vS which occurred during the forward rectification mode. However, with the spike 16 having negative polarity impinging upon the secondary winding 8, the diode 14 is reversely biased. At the same time, as is well known in the art, inductors always maintain current continuity and attempt to sustain the original current flows. Thus, with the reverse-biased diode 14 acting as an open circuit, the stored energy in the windings 6 and 8 goes nowhere but as spurious current charging the parasitic elements in its path. The current discharge is in the form of a damped oscillation until all the stored energy is dissipated. The converter 2 is said to be in a resetting mode. The current path of the resetting mode is identified by the reference numeral 13.
In the same manner as the windings 6 and 8 of the transformer 5, at the end of the half-cycle at t=t1, the stored energy in the inductor 10 also releases itself. In this case, the discharge is through the capacitor 12 and the freewheel diode 15. The inductor 10 is normally designed with a large inductive value. The freewheeling current normally continues until the onset of the next switching cycle. The path of the freewheeling current flow is identified by the reference numeral 17 shown in FIG. 1. The converter 2 is said to be in a freewheeling mode.
Attention is now directed to the rectifier 9 in FIG. 1. The diode 9 poses considerable Ohmic drop during the forward rectification mode. In operation, the p-n junction of the diode 9 can consume approximately 0.7 Volt of voltage level. To rectify this shortfall, attempts have been made to insert a Schottky diode 18 as a replacement for the regular diode 14, as shown in FIG. 3. Still, the Schottky diode 18 can adsorb close to 0.5 Volt of voltage level.
An efficient design of the converter 2 is to have the resetting current totally discharged swiftly and efficiently with minimal disturbance to the normal operation of the entire circuit 2. A slow decay of the resetting current in comparison to the switching frequency of the switch 7 can distort the periodic waveform feeding the primary winding 6, causing the “staircase-DC-bias” effect. The staircase-DC-bias effect is to be avoided and is especially crucial in modern-day switching mode power converter with compact sizes operating at high frequencies. There is still another undesirable effect for not efficiently discharging the resetting current. Specifically, if the resetting current is discharged through a high-impedance discharge path, excessive Joule heat can be generated. The generated heat not only undercuts the power efficiency by unnecessarily consuming power as wasteful heat. Excessive heat generated, if not properly controlled, can also detrimentally effect reliability.
Modern-day converter designs require compactness, low power consumption, and efficiency. For special applications such as high-speed data communications and computing, circuits are operated at very low voltage levels yet demanding high current outputs. Too high a voltage drop consumed by the converter is undesirable and sometimes impractical. To further curtail the Ohmic drop, FETs (Field Effect Transistors) have been adopted to substitute the diodes in the rectifying circuit 9. As shown in FIG. 4, a FET 20 is disposed to take the place of the diode 14. However, the FET 20 must be controlled by a control circuit 22 to provide proper timing signals to the FET 20 such that the FET 20 turns on and off appropriately. That is, the control circuit 22 has to operate in synchronization with the timing of the switch 7 (FIG. 1). Accordingly, the rectifier 9 shown in FIG. 4 is called a synchronous rectifier, and the process is called synchronous rectification. Due to the various operating modes as mentioned above, the control circuit 22 must operate with precise timing. If the FET 20 is turned on incorrectly, a circuit short may occur. Likewise, if the FET 20 is turned off at the wrong time, a unacceptable high voltage drop may result causing significant decline in operating efficiency and overheating.
The converter 2 shown in FIG. 1 is a single-ended converter. For usage at high power levels, double-ended circuit schemes, such as push-pull, half-bridge, and full-bridge designs are common. Because of the relatively complex current traffic of the various modes of operation of the converter 2 as described above, providing a control circuit 22 with proper timing is quite elaborate. Heretofore, there has not been any practical scheme that works satisfactorily.
In light of the above, there is a need to provide efficient switching-mode power converters utilizing synchronous rectification.